This invention relates to the field of stress and strain measurement, specifically to an improved method for detecting and viewing of stress and strain in objects or parts.
Stress and strain detection and measurement is an important field of engineering and is used in almost every area of manufacture and construction where a knowledge of the stresses and strains being experienced by an object are important. By knowing the stresses in a part, failure modes and service life can be predicted and failure analysis can be performed. With this knowledge, parts can be redesigned to be lighter, stronger, or less expensive. xe2x80x9cStressxe2x80x9d and xe2x80x9cstrainxe2x80x9d are sometimes used interchangeably in the following descriptions, since one can be determined if the other is known and a stress-strain diagram is available.
Strain, e, is a dimensionless response to stress expressed as a fraction e=xcex94L/Lo where Lo is the original length of the object and xcex94L is the change in length of the object when stress is applied. Stress, s, is a measure of force per unit area given by F/A where F is the force being applied and A is the area it is being applied to. Because stress cannot be measured directly in practice, strain is measured instead. The stress in an object is related to the strain by the Young""s Modulus, E, which is given by the following relationship:   E  =            s              e        el              =                            (                      F            /            A                    )                          (                      Δ            ⁢                          xe2x80x83                        ⁢                          L              /                              L                o                                              )                    =              stress        /                  strain          .                    
Knowing the Elastic Modulus of a given material, the stress in the material can be determined by measuring the strain. Traditionally, stress and strain measurements have been accomplished by a number of different methods. Some of these methods are described below:
Strain gages are small electronic devices that measure strain through a change in resistance. The resistance, R, of a wire is a function of the size of the wire as well as of the material as follows:       R    =          ρ      ⁡              (                  L          A                )              ,
where L is the length of the wire, A is the cross-sectional area of the wire, and xcfx81 is electrical resistivity, a property of the material. As the length of the wire L increases and the cross-sectional area A decreases, the resistance R increases. This property can be exploited to measure strain with a strain gage. By measuring the increase in resistance of a length of a thin wire attached to a part, the strain in the part can be determined and the stress calculated.
Unfortunately, strain gages have a number of disadvantages. First, applying a strain gage to a part can be difficult. Second, the electrical signal produced by a strain gage is very small and must be amplified. Amplification can lead to noise problems and loss of accuracy. Another significant disadvantage of strain gages is they can only measure strain in one direction. A different strain gage must be used for every different direction in which strain is to be measured. Finally, strain gages can only measure localized strain. That is, the strain gage can only measure strain exactly at the point where the gauge is applied. As such, strain gages require prior knowledge of the stress and strain distribution in the part and the direction of strains in order to be most effective.
Brittle lacquer is a brittle coating that cracks easily under tensile strain. The lacquer is applied to the unstressed part. When the part is stressed, the brittle lacquer cracks, starting at the areas of highest strain. Brittle lacquer is difficult to work with and does not provide a quantitative measure of the stress and strain. As such, the brittle lacquer method can only indicate which areas of a part are experiencing stress and strain. Also, only one test is possible with a given application of brittle lacquer. Once the brittle lacquer has cracked, the coating must be stripped off and reapplied for subsequent tests.
Fiber-optics can be used to measure stress and strain by detecting the change in length of all optical fiber. In theory, the operation of a fiber-optic strain gage is similar to the operation of a strain gage that measures change in resistance. In the case of a fiber-optic strain gage, a change in the transmissibility of light is being measured. Fiber-optic strain gages possess the same disadvantages as standard strain gages: they are difficult to apply and can only measure localized strain in one direction. As such, prior knowledge of the stress and strain field in the part is required.
Because of the disadvantages and the complexity of strain gages, brittle lacquer, and fiber-optics, these techniques for measuring stress and strain are typically used only at the product development stage. Manufactured products generally do not come with built-in strain gages for monitoring stresses and strains, although this might be desirable in some cases. For instance, monitoring the stresses and strains in a production aircraft part would be useful to help predict failures of that part and to schedule maintenance on that part. Moreover, the cost of these particular methods of detecting stress and strain make them somewhat prohibitive even at the product development stage.
A different class of stress and strain measurement techniques which have been used for a number of years are known as photoelastic techniques. Photoelastic techniques exploit the photoelastic properties of certain materials to detect stress and strain. The speed of propagation of light in transparent materials is generally slower than in a vacuum or in air. The ratio of the speed of light in a given material to the speed of light in a vacuum is called the index of refraction of that material. In homogeneous materials, the index of refraction is constant regardless of the direction of propagation or plane of vibration of the light. In other materials, strain in the material causes the index of refraction to change depending on the direction of propagation of light. These materials, which can be optically isotropic when unstrained, become optically anisotropic when strain is present.
Materials which become optically anisotropic when stressed are known as photoelastic materials. The change in index of refraction relative to index axis in the material can typically be related to the stress and strain in the material by observing and quantifying the photoelastic effect. The photoelastic effect is caused by alternately constructive and destructive interference between light rays which have undergone relative retardation, or phase shift, in the stressed photoelastic material. When illuminated with polarized light and viewed through a polarizing filter, fringe patterns become visible in the photoelastic material that reveal the overall stress and strain distribution in the part and show the locations and magnitudes of the stresses and strains in the part. A person skilled in the art of photoelastic analysis can interpret and measure these patterns.
Photoelastic techniques have the advantage of being a full-field measurement technique. The strain over the entire surface of the part can be measured. Furthermore, the measurement technique is not directional. Unlike strain gages, a photoelastic coating can detect strain regardless of the direction of that strain. As such, prior knowledge of the directions and magnitude of the strain in the part before applying the photoelastic coating is not required.
Photoelastic techniques are an excellent technique for stress and strain analysis with many advantages over other methods of analysis. However, the application of photoelastic coatings to parts is problematic. Presently, photoelastic materials are available in sheets and plate form for application to flat parts. To perform the analysis, a sheet of photoelastic material must be carefully cut to shape and bonded to the part. The part is then subjected to test forces. While being subjected to the test forces, the part can be viewed through a reflection polariscope to determine the direction and magnitude of the stresses and strains. For photoelastic analysis on more complex parts, a viscous liquid is used which is cast on a flat-plate mold. While still in pliable state, the sheet is removed from the mold and must be formed to the part by hand. Finally, the sheet must be bonded to the part with a reflective cement.
Photoelastic analysis using existing photoelastic coatings and methods of application have numerous disadvantages. These disadvantages include the following:
1. Photoelastic coatings are custom materials that much be specially purchased. Photoelastic coatings have limited availability and are costly because photoelastic analysis is such a specialized field.
2. The process of applying photoelastic coatings is labor intensive and imprecise. The process often involves cutting and fitting the photoelastic coating to the part by hand.
3. Photoelastic material in the form of flat sheet and plate can only be used on two-dimensional parts or very simple three-dimensional parts.
4. Molding photoelastic material to three-dimensional parts using viscous liquids is a very labor-intensive process.
5. Achieving a perfect fit between the photoelastic coating and a part with even a slight amount of complexity is virtually impossible.
6. Once the photoelastic coating has been shaped, it must be bonded to the part in a separate operation with a special reflective cement.
7. Existing methods of applying photoelastic coatings are not easily automated.
8. Traditional photoelastic coatings are only used on test parts and are not suitable for use on production parts and mass-manufactured parts.
9. Traditional photoelastic coatings do not provide protection for the part nor aesthetic enhancement of the part.
Although photoelastic techniques offer numerous advantages over other techniques for detecting stress and strain, they are limited by the difficulty and cost of applying photoelastic coatings, particularly on more complex three-dimensional parts. Because of the labor-intensive nature of applying photoelastic coatings, present photoelastic techniques have limited applicability, especially to mass-manufactured parts. There is a need for a new method of detecting and providing stress and strain information easily and cost-effectively, especially on three-dimensional parts and on mass-manufactured parts.
In a field seemingly unrelated to stress and strain detection and measurement, powder coating is an advanced finishing process widely used in different industries. Powder coating is typically defined as any coating applied as a dry (without solvent or other carrier), finely divided solid which adheres to the substrate as a continuous film when cured by heating or fusing.
Powder coating typically involves applying plastic in powder form to a part and then curing the powder in an oven so it fuses together and adheres to the surface of the part. Most powder coats are electrostatically applied although different methods of applying the powder are also possible. Different types of plastics typically used for powder coating include polyesters, epoxies, acrylics, urethanes, and various hybrid blends. Some powder coats include chemicals such as TGIC (Triglycidyl Isocyanurate) to increase the durability of the coating.
A powder coat finish can be applied to a variety of different materials. While originally developed for metals, some recent advances in powder coating technology allow for the application of a powder coat finish to some ceramics, woods, and plastics. Powder coating is a widely used protective coating often applied at the time of manufacture directly to a variety of products and parts for protection and aesthetic enhancement.
Powder coating can provide a durable and attractive finish. Powder coating has numerous advantages. Powder coating is an economical, long-lasting, and color-durable finish. Powder coated surfaces are typically more resistant to chipping, scratching, fading, and wearing than many other finishes. A wide range of colors are available in high and low gloss, metallic, and clear finishes. Texture selections range from smooth to rough finishes designed for hiding surface imperfections. Powder coating is lightweight and non-toxic. In addition, the powder coating process results in less waste than commonly found in liquid finishing processes.
A wide range of products and parts covering a broad spectrum of industries are currently powder coated. These products include appliances, automotive applications, window frames, light poles, bicycles, tractors, shelves, cabinets, baby strollers, just to name a few. To date, powder coat has typically been used for providing a protective and aesthetic finish.
In accordance with the present invention, a new method of detecting and measuring stress and strain is described comprising the use of a powder coat finish and photoelastic techniques. A test part is provided with a non-opaque layer of powder coat which becomes optically anisotropic when subjected to stress and strain. Stress and strain in the part will cause fringe patterns in the powder coat finish that can be viewed and measured in the non-opaque layer of powder coat using photoelastic techniques. Stresses and strains in the part can be detected and measured when the part is subjected to various forces. Alternatively, permanent structural deformation can be detected and measured after the part has experienced plastic deformation.